Method and apparatus for scan line skew correction using a gradient index electrooptic prism

Abstract

An optical element for the selective scan line skew correction in a raster output scanner (ROS) of an electrostatographic printer. The optical element includes a chamber having a volume defined by parallel surfaces in substantially parallel planes containing a liquid crystal material having a variable index of refraction as a function of voltage applied thereacross. An inhomogenous electric field is generated through the liquid crystal material to produce a refractive index gradient along substantially perpendicular planes in the optical element. The ROS includes a control and feedback system coupled to a variable voltage source for applying a variable biasing voltage across the optical element to control the variable index of refraction of the liquid crystal material for selectively correcting scan line skew.

We claim: 
     
       1. An optical element for selective skew correction of a scanning light beam transmitted therethrough, comprising: a member including a pair of optically transmissive plate members defining a chamber therebetween;   a liquid crystal material substantially filling the chamber, said liquid crystal material having a variable index of refraction responsive to an electric field therethrough; and   means for generating an inhomogenous electric field through said liquid crystal material to produce a refractive index gradient across said optical element.   
     
     
       2. The optical element of claim 1, wherein said electric field generating means includes means for producing a refractive index gradient along two substantially perpendicular planes. 
     
     
       3. The optical element of claim 1, wherein said electric field generating means includes a resistive array having a plurality of resistive strips situated on at least one of said pair of optically transmissive plate members. 
     
     
       4. The optical element of claim 3, wherein said resistive array includes: first and second resistive strips extending parallel to one another; and   a plurality of resistive strips extending between said first and second resistive strips, substantially perpendicular thereto.   
     
     
       5. The optical element of claim 1, wherein said electric field generating means includes an electrode grid having a plurality of individually controllable electrodes. 
     
     
       6. The optical element of claim 1, wherein said plate members include a pair of substantially planar parallel external surfaces having a substantially uniform thickness therebetween. 
     
     
       7. The optical element of claim 1, further including means, coupled to said electric field generating means, for applying a biasing voltage across said member to produce an electric field through said liquid crystal material such that the index of refraction of said liquid crystal material is varied as a function of the applied biasing voltage. 
     
     
       8. The optical element of claim 1, wherein said material having a variable index of refraction includes a nematic liquid crystal material. 
     
     
       9. A raster output scanner for providing selective skew alignment correction, comprising: means for generating a light beam modulated in accordance with a video input data signal;   means for scanning the light beam onto a movable imaging surface being transported in a process direction to provide successive scan lines on the movable imaging surface;   an optical element for selectively deflecting the light beam to correct scan line skew on the imaging surface as a function of voltage applied thereacross, including a member including a pair of optically transmissive plate members defining a chamber therebetween;   a liquid crystal material substantially filling the chamber, said liquid crystal material having a variable index of refraction responsive to an electric field therethrough; and   means for generating an inhomogenous electric field through said liquid crystal material to produce a refractive index gradient across said optical element.     
     
     
       10. The raster output scanner of claim 9, further including a voltage source for applying a biasing voltage across said optical element to generate the electric fields through said liquid crystal material such that the index of refraction of said liquid crystal material is varied as a function of the applied biasing voltage. 
     
     
       11. The raster output scanner of claim 9, wherein said electric field generating means includes means for producing a refractive index gradient along two substantially perpendicular planes. 
     
     
       12. The raster output scanner of claim 9, wherein said electric field generating means includes a resistive array having a plurality of resistive strips situated on at least one of said pair of optically transmissive plate members. 
     
     
       13. The raster output scanner of claim 12, wherein said resistive array includes: first and second resistive strips extending parallel to one another; and   a plurality of resistive strips extending between said first and second resistive strips, substantially perpendicular thereto.   
     
     
       14. The raster output scanner of claim 9, wherein said electric field generating means includes an electrode grid having a plurality of individually controllable electrodes. 
     
     
       15. The raster output scanner of claim 9, wherein said plate members include a pair of substantially planar parallel external surfaces having a substantially uniform thickness therebetween. 
     
     
       16. The raster output scanner of claim 9, wherein said material having a variable index of refraction includes a nematic liquid crystal material. 
     
     
       17. The raster output scanner of claim 9, further including: means for detecting scan line position on the imaging member to generate an error signal in response thereto; and   means, responsive to the error signal, for controlling the biasing voltage applied to the optical element for selectively varying the index of refraction thereof.   
     
     
       18. An electrostatographic printing machine of the type in which a raster output scanner produces a modulated light beam wherein the modulated light beam is scanned across a movable photoconductive imaging surface being transported in a process direction for recording latent images thereon, comprising: means for generating a light beam modulated in accordance with a video input data signal;   means for scanning the light beam onto a movable imaging surface being transported in a process direction to provide successive scan lines on the movable imaging surface;   an optical element for selectively deflecting the light beam to correct scan line skew on the imaging surface as a function of voltage applied thereacross, including a member including a pair of optically transmissive plate members defining a chamber therebetween;   a liquid crystal material substantially filling the chamber, said liquid crystal material having a variable index of refraction responsive to an electric field therethrough; and   means for generating an inhomogenous electric field through said liquid crystal material to produce a refractive index gradient across said optical element.     
     
     
       19. The electrostatographic printing machine of claim 18, further including a voltage source for applying a biasing voltage across said optical element to generate the electric fields through said liquid crystal material such that the index of refraction of said liquid crystal material is varied as a function of the applied biasing voltage. 
     
     
       20. The electrostatographic printing machine of claim 18, wherein said electric field generating means includes means for producing a refractive index gradient along two substantially perpendicular planes. 
     
     
       21. The electrostatographic printing machine of claim 18, wherein said electric field generating means includes a resistive array having a plurality of resistive strips situated on at least one of said pair of optically transmissive plate members. 
     
     
       22. The electrostatographic printing machine of claim 21, wherein said resistive array includes: first and second resistive strips extending parallel to one another; and   a plurality of resistive strips extending between said first and second resistive strips, substantially perpendicular thereto.   
     
     
       23. The electrostatographic printing machine of claim 18, wherein said electric field generating means includes an electrode grid having a plurality of individually controllable electrodes. 
     
     
       24. The electrostatographic printing machine of claim 18, wherein said plate members include a pair of substantially planar parallel external surfaces having a substantially uniform thickness therebetween. 
     
     
       25. The electrostatographic printing machine of claim 18, wherein said material having a variable index of refraction includes a nematic liquid crystal material. 
     
     
       26. The electrostatographic printing machine of claim 18, further including: means for detecting scan line position on the imaging member to generate an error signal in response thereto; and   means, responsive to the error signal, for controlling the biasing voltage applied to the optical element for selectively varying the index of refraction thereof.